The present invention generally relates to cellular mobile radio communication. More specifically, the invention relates to methods and systems for efficient and flexible use of the frequency spectrum available for communication in a frequency division multiple access (FDMA) or a time division multiple access (TDMA) mobile radio communication system. The present invention also relates to base stations and mobile stations for flexible and efficient use of the frequency spectrum available in such communication systems.
Many mobile radio telephone systems of various kinds are known and in use. In all of these systems, the frequency band available for connections limits the number of possible simultaneous connections, or capacity, of the system. Two base stations or mobile stations transmitting on the same radio channel of an FDMA system or on the same time slot of the same radio channel in a TDMA system may interfere with each other. This kind of interference is sometimes called co-channel interference because the interference comes from the same radio channel. If the signal strength of the signals relating to one of the connections is not sufficiently strong relative to the strength of the interfering signals, the information on the first connection will then be more or less unintelligible. If the interfering mobiles or base stations are sufficiently distant from each other, however, the signals relating to the connection will be sufficiently stronger than the interference signals and the information of the connections will be received and properly decoded.
In order to be able to use the same radio channel in FDMA systems, or the same time slot of a radio channel in TDMA systems, for more than one connection, some mobile radio systems are set up as cellular systems. The geographical area to be covered by such a system is divided into smaller areas, called cells, and mobiles in a cell communicate with a base station for that cell. Some or all of the available radio channels are distributed among the cells according to a frequency plan.
A conventional frequency plan provides that different radio channels are allotted to a cluster of adjacent or neighboring cells. No two cells in the: same cluster can use the same radio channel at the same time. Each radio channel used by the base station or a mobile station of one cell in a cluster, is different from every channel used by a base or mobile in another cell in the same cluster. However, cells in different clusters may use the same radio channels. Thus there can be simultaneous multiple use of a radio channel. Such multiple use is sometimes called channel or frequency m-use. The distance between cells using the same radio channel is known as the m-use distance.
Many different shapes and sizes of cell clusters are known to those skilled in the art, e.g. 3-cell, 4-cell, 9-cell, 12-cell and 21-cell clusters. Somewhat simplified, the largest call handling capacity for a cellular system is achieved when using the smallest type of cluster that provides sufficiently low co-channel interference.
Although the frequency plans described above provide the import:ant advantage of plural use of radio channels, such fixed frequency plans are cumbersome to implement. Due to geographical variations, the cells covered by each base station antenna will vary in size and shape. The coverage area of the system will thus normally be covered by several different combinations of the known cluster combinations. Commonly, the cluster configuration, or decisions of which re-use patterns to be used, are made using complex computer simulations of the topography in the system.
Other disadvantages are also inherent in the use of fixed frequency plans. Normally, the number of desired connections in a cell varies with time and one cell may not be able to handle all desired connections because all of the channels and all of the time slots on TDMA channels allotted to the cell are occupied. At the same time the number of desired connections in an adjacent cell, or any cell in the same cluster, may be substantially less than the total capacity on all channels allotted to that cell according to the fixed frequency plan. Thus all desired connections cannot be handled by the cell cluster despite the fact that there is at least one free channel or at least a free time slot on a radio channel which could have been used for the desired connections had this not been forbidden by the fixed frequency plan.
One way of reducing the above mentioned disadvantage of fixed frequency plans is to distribute some of the radio channels available for connections in a mobile radio communication system, and to keep some radio channels in reserve. All of the channels except for the reserved channels are distributed according to a frequency plan. The reserved radio channels may be temporarily used by any cell requiring additional capacity above that provided by the channels permanently allotted to that cell in accordance with the frequency plan. Such temporary use of a reserved channel is subject to not causing co-channel interference with a connection in another cell already using that reserved radio channel. While this method of reserving and temporary allotting some radio channels provides more flexibility as regards variable connection handling capacity than a fixed frequency plan, the total handling capacity for the whole system may decrease.
A more profound method of obtaining high traffic handling flexibility in various areas of a cellular mobile radio system is to completely abolish the fixed frequency plan in favor of letting all radio channels be available for connections in all cells. Any cell may use any radio channel available for connections, provided there is sufficiently low co-channel interference from other cells currently using the same radio channel. This method of using the available radio channels is sometimes called dynamic channel allocation (DCA). While this method certainly affords advantages as regards changing call handling capacity for a cell, it also includes disadvantages. For example, DCA is relatively complex since it requires many quality measurements made by mobile stations that are reported to base stations. Moreover, DCA also involves frequent handovers.
In conventional FDMA or TDMA systems where the same radio channel is used throughout a connection, any co-channel interference will last as long as both the connections last when the transmissions occur more or less simultaneously on the same radio channel. Thus, a worst case situation must be considered in frequency planning and cell cluster design to ensure that the minimum acceptable signal quality is maintained. Frequency hopping is a technique for ensuring that worst case interference scenarios do not prevail for longer than one frequency hop interval as opposed to the duration of the entire connection.
In a frequency hopping system each cell can use all of the available channels, but at different times, as determined by a pseudo-random frequency hop sequence generator. Such generators can be constructed either to yield a random probability that any two cells choose the same frequency at the same time (known as non-orthogonal hopping), or to guarantee that specified cells or mobile stations never choose the same frequency at the same time (known as orthogonal hopping) or a mixture of the two techniques (e.g., signals in the same cell hop orthogonally, while being non-orthogonal relative to adjacent cell signals). Today there is only one commercial example of a frequency hopping cellular radio system. The European GSM standard describes this system, which is based on a combination of time division multiple access (TDMA) in which a 4.6 mS time cycle on each frequency channel is divided into eight, 560 .mu.S time slots occupied by different users, and frequency hopping in which the frequency of all eight time slots changes every 4.6 mS.
There are, however, several drawbacks associated with frequency hopping systems, in general, and the GSM standard in particular. For example, the occasional loss of data in a frequency hopping system can be ameliorated by providing redundant information bits which the demodulator in a receiver can use to recover correct information. Typically, this is accomplished in frequency hopping systems which provide error correction coding that spreads redundant information bits over a number of frequency hops by interleaving. Such systems work best when data is interleaved over many hops rather than just a few hops, but an undesirable side-effect of interleaving is a corresponding increase in transmission delay.
Along the same lines, it has been recognized to be advantageous in frequency hopping systems, such as GSM, that adjacent base stations have their TDMA frame structures, and therefore their frequency hopping instants, synchronized, as this facilitates the handover of communication with a mobile station from one base station to another base station as the mobile station crosses the boundary between their coverage zones. If adjacent stations are not synchronized, then the mobile station must somehow obtain knowledge of the timing of the new base station, ideally before relinquishing contact with the original base station, a task which is technically difficult to implement.
Moreover, requiring synchronization between adjacent base stations necessitates synchronization of an entire, nationwide network and there are difficulties in coordinating this when the stations even within a single country are owned by a multiplicity of competing service providers. Therefore, the GSM standard does not specify synchronized base stations and individual network operators can, for example, choose to synchronize the stations within their own network while being unsynchronized in other networks.